1. Field of the Invention
The present invention relates to exhaust gas purifying apparatus for purifying exhaust gas discharged from an internal combustion engine.
2. Description of the Related Art
In an internal combustion engine mounted on an automotive vehicle or the like, it is necessary to purify discharged exhaust gas, such as components of, for example, carbon monoxide (CO), nitrogen oxide (NOx), or hydrocarbon (HC) before being discharged into the atmosphere.
In particular, it is important to purify the unburnt gas components discharged starting of the internal combustion engine. In this case, when starting the internal combustion engine, an air/fuel ratio of mixture is set at a low air/fuel ratio (on an enriched side) in comparison with a stoichiometric air/fuel ratio in order to enhance startability of the internal combustion engine. However, since the temperature of the internal combustion engine is low and the combustion is unstable, a large amount of the unburnt gas components, such as the unburnt hydrocarbon, is discharged.
To meet this demand, an "engine exhaust gas purifying apparatus" described in Japanese Patent Application Laid-Open No. Hei 6-33747 is well known. In this apparatus, an adsorbent for adsorbing unburnt hydrocarbon (HC) contained in the exhaust gas below a predetermined temperature and for releasing the adsorbed unburnt hydrocarbon (HC) at a temperature equal to or higher than the predetermined temperature, is provided at an exhaust passage upstream of a three way catalyst, and an electric heated catalyst (EHC) is provided in the exhaust passage between the adsorbent and the three way catalyst.
In such an exhaust gas purifying apparatus, in the case where the engine is started in a cold condition so that the three way catalyst is kept under an inactive condition, the unburnt hydrocarbon (HC) discharged in starting the engine is adsorbed onto the adsorbent and current is fed to a heater of the electric heated catalyst to thereby activate the catalyst.
Thereafter, when the adsorbent is subjected to heat of the exhaust gas and reaches a predetermined temperature, the unburnt hydrocarbon (HC) adsorbed on the adsorbent is released away from the adsorbent and is caused to flow into the electric heated catalyst. However, at this time, since the electric heated catalyst is activated by the heater, the above described unburnt hydrocarbon (HC) is purified by the electric heated catalyst.
However, in the case where the above described exhaust gas purifying apparatus is applied to an internal combustion engine having a plurality of adsorbents which are provided at parallel exhaust passages upstream of the electric heated catalyst, an unburnt hydrocarbon (HC) is adsorbed on each adsorbent when starting the internal combustion engine. When a temperature of the adsorbents are elevated to a predetermined temperature, the unburnt hydrocarbon (HC) adsorbed on each adsorbent is caused to be introduced into the electric heated catalyst.
In this case, there are differences of position of the adsorbent in the exhaust passages and of temperature of the exhaust gas introduced into the adsorbents. As a result, the unburnt hydrocarbon is introduced into the electric heated catalyst from each adsorbent at different times. This causes the unburnt hydrocarbon, which have been desorbed from the adsorbent, to be introduced into the electric heated catalyst for too long of a period. In other words, since an air/fuel ratio is rich in the electric heated catalyst for a long time, the temperature of the electric heated catalyst is dropped below the activated temperature. Accordingly, in order to maintain the activated temperature of the electric heated catalyst, it is necessary for current to be fed to the heater for a long time. Thus, electric power in a battery is wasted. Therefore, it is necessary to enlarge battery capacity.
In another case where the above described exhaust gas purifying apparatus is applied to an internal combustion engine having a plurality of adsorbents which are provided in parallel exhaust passages upstream of the three way catalyst without a heater, in warming up the internal combustion engine, a rich air/fuel ratio gas of unburnt hydrocarbon is discharged from the engine. At the same time, the unburnt hydrocarbon is desorbed from the adsorbents when a temperature of each adsorbent is elevated to a predetermined temperature. As a result, since a large amount of the unburnt hydrocarbon is introduced into the three way catalyst for a long time, the temperature of the three way catalyst is dropped below the activated temperature. Accordingly, there is a possibility that the unburnt hydrocarbon could be discharged into the atmosphere without completely purifying the exhaust gas through the catalyst.